1. Technical Field
The present invention relates to a pneumatic tire and a molding die thereof.
2. Related Art
Snow tires (studless tires), for example, have incisions called sipes which are made in land portions, such as blocks and ribs. An edge effect by the sipes enables stable running on a road surface covered with snow and ice where a frictional coefficient is low. Whereas the sipes obtain the effect as above, as is shown in FIG. 9, a sipe 101 lowers rigidity of a land portion 102 and the land portion 102 undergoes deformation (collapses) to a greater extent when making contact with the ground. Such deformation may possibly reduce the edge effect contrary to the intention or lower resistance to irregular wear.
In order to suppress deformation of the land portions when making contact with the ground, increasing frictional resistance between a pair of opposing wall surfaces of a sipe by providing 20 to 300 μm irregularities to a pair of the wall surfaces (see JP-A-8-175115) or narrowing a groove width of a sipe has been proposed.
As a method of narrowing the groove width of a sipe, for example, JP-A-11-170818 discloses that a groove width of a sipe is made narrower in an intermediate portion in a sipe length direction than at both ends. JP-A-11-42913 discloses that a groove width of a sipe is set to 0.1 to 0.3 mm, which is narrower than a typical groove width, and also to provide a pillar-like space extending in a sipe depth direction to reinforce a sipe plate. JP-A-2-241806 and JP-A-2-303908 disclose to provide a sipe with a narrow portion or a knife cut portion having a narrow groove width and a wide portion having a wide groove width and surrounding the narrow portion or the knife cut portion at least partially.
When a sipe having a narrow groove width as above is provided, a sipe plate to shape the sipe becomes thinner, which raises a concern about breakage of the sipe plate over repetitive use for molding of a tire by vulcanization. In order to enhance a breakage suppressing effect on the sipe plate, it is effective to surround a thin plate portion of the sipe plate with a thick portion as are described in JP-A-2-241806 and JP-A-2-303908. However, when the thin plate portion is surrounded by providing a thick portion at a tip end of the sipe plate (that is, when a sipe is provided with a wide portion having a wide groove width at a sipe bottom and a narrow portion having a narrow groove width on top of the wide portion), problems as follows are raised. That is, in this case, resistance of the sipe plate is large when pulled out from the surface of tread rubber at the time of die releasing of a tire (that is, when a tire molded by vulcanization is released from the molding die), and large resistance causes chipping of rubber.